In both exocytosis and viral fusion, the first recordable event is a fusion pore. We have measured fusion pores in both beige mouse mast cells during the exocytotic release of histamine, as well as viral induced cell-cell fusion. We observe a pore which widens quickly for 10 milliseconds, then exhibits a variable pattern of widening and contracting before finally opening rapidly to complete fusion. The experimental behavior is explained in a molecular model where the fusion pore is composed of lipid and protein. This model proposes that the hydration barrier to membrane fusion is overcome by inter-membrane protein adhesion, leaving pockets of trapped water between strained lipids. The lipids rearrange, coat the membrane fusion proteins, and a pore forms. This initial pore subsequently widens by accretion of proteins and lipids. To understand the forces which govern this reaction, we are studying protein hydration, protein/lipid interaction, and pore kinetics in well defined systems where we have better experimental control of thermodynamic parameters, and can measure the energetics of surface hydration. The presence of carbonyls is not essential for lipid structure and hydration. While the dipole potential of bilayer lacking carbonyls is 118 mV lower than that of bilayers with carbonyls, the two lipids have identical hydration forces. These forces are implicated in channel gating, where 40-50 water molecules leave the channel as it closes.